The present invention relates to a manufacturing method of a flexible amorphous silicon solar cell.
Solar cell is a new pollution-free energy source with applications and market demand thereof increasing steadily. It finds applications in consumer electronics products as well as in power system, and has good prospects of being one of the major energy sources. Flexible amorphous silicon solar cells are fabricated by depositing amorphous silicon, metallic and transparent conductive oxide films on a flexible substrate with the desirable features of light weight, thin thickness, flexibility, portability and nonfracture, and can be more useful than the traditional glass-substrate amorphous silicon solar cells.
There are two main types of flexible amorphous silicon solar cells, namely:
(1) The ones with metal substrate
Presently, the substrate is made of stainless steel on which there are sequentially grown metallic, amorphous silicon, and transparent conductive oxide (TCO) films, as typified by the products of ECD, Sovonics Solar System companies of the U.S.
(2) The ones having the substrate made of a polymer material
Two kinds of polymer films are used, including films of polymer materials with good transparency and those with poor transparency. The former has a very high transmission in the visible light range so that there can be sequentially grown thereon transparent conductive oxide, amorphous silicon and metal films to form a structure the same as that of the glass-substrate solar cell purchased from Sanyo Electric Co. Ltd. Amorphous silicon solar cells with the substrate made of a polymer material with poor transparency have the same structure as that of metal substrate amorphous silicon solar cells. On the amorphous silicon solar cells, there are sequentially grown metal amorphous silicon, and transparent conductive oxide films, as typified by the solar cells developed by the Iowa Thin Film Technology Inc. of the U.S., and the Sanyo Electric Co., Ltd. of Japan.
Compared with polymer substrate solar cells, metal substrate solar cells are heavy and not easily rollable. This is because it is not easy to make metal substrate thin and rollable enough. The flexible solar cells are required to be light-weight, thin as well as flexible. Although European Patent Application No. 0,109,976 filed by Sovonics Solar System has disclosed to make a very thin, light weight and flexible amorphous silicon solar cell on a very thin metal substrate (&lt;50.mu.m), or by etching away the metal substrate after the cell is produced, the manufacturing procedure thereof is too complicated, and the cost is too high. Solar cells with polymer substrate will be more flexible, lighter in weight, and thinner than those with metal substrate.
The polymer films suitable for being used as the substrate of solar cells should have the following characteristics:(1) good heat stability; (2) good mechanical strength; (3) surface smoothness and thickness uniformity; (4) high purity (low ion content) to avoid gas release in a high vacuum, and (5) good weatherability to avoid degrading after long time exposure to sunlight. Therefore, developing a suitable polymer material for the substrate would be one of the key techniques. Besides, the choice of the right technology for depositing the films on polymer substrate in vacuum is also critical because of the significant thermal expansion coefficient of polymer films. The right technology would ensure that the polymer substrate would not be deformed or warped, and the films deposited thereon would be even in thickness and would not be easily stripped off or cracked.
The types of polymer substrates for polymer film solar cells include:
1) polyimide (PI)-film substrate:
Most of the PI films used are those marketed under the trademark of "Kapton" from Du Pont of the U.S., having primary constituents of pyromellitic dianhydride (PMDA) and oxydianiline (ODA), and with desirable characteristics in heat resistance, mechanical strength, electrical insulation, radiation-shielding, and chemical-resistance. However, its coefficient of thermal expansion (TCE) is relatively high (4.0-5.0.times.10.sup.-5 cm/cm.degree. C.) and the PI film is very soft, and when aluminum (TCE:1.3.times.10.sup.-5 cm/cm.degree. C.), chromium (TCE:6.times.10.sup.-6 cm/cm.degree. C.), and amorphous silicon (TCE:1.9-4.times.10.sup.-6 cm/cm.degree. C.) films are deposited on the PI film in vacuum (with operation temperature up to 250.degree. C.), there would be a stress mismatch between the PI film and the films deposited on it, resulting in the deposited films being easily stripped off and cracked, and uneven in thickness.
To alleviate the problems of warping and excessive thermal expansion coefficent of PI films used as substrate, some people tried to add a supporting layer under the PI film as disclosed in U.S. Pat. No. 4,514,583; Japan kokai Nos. 60-79779 and 60-66869; and European Patent Application No. 0 ,189,976 filed by Sovonics Solar System. This supporting layer is mostly a thin film of a metal. Some use fabrics or polymer material. The European Patent Application No. 0,189,796, filed by Sovonics Solar System, uses a very thin metal film on which a PI film is applied. When the films are deposited on the substrate, the supporting layer is selectively etched to get the flexible solar cell. Alternatively, the metal film can be retained as a part of the substrate. The method according to the Sovonics Solar System patent is too complicated. The composite substrate comprising a thin metal supporting layer and a PI film is still too soft, and it's thermal expansion coefficient would still be excessive, and the substrate would be warped during the a-Si:h deposition.
Supporting layer made of fabrics or glass fiber fabric would not make a good substrate since it has a surface having a roughness possibly amounting to several .mu.m to several ten's .mu.m and the PI film applied thereon will also be uneven.
2) Polyether sulfone (PES) film-substrate:
PES has a glass transition temperature of 225.degree. C. and will easily release gas in a vacuum. Thus PES substrate (TCE) has poor characteristics and is not suitable for high efficiency device. The TCE of PES is 5.5.times.10.sup.-5 cm/cm.degree. C. which is even larger than that of PI and presents more difficulty in reaching the stress match between PES film and metal, amorphous silicon films and indium tin oxide (ITO, a transparent electrode material).
3) PI/MeO/PI/MeO (wherein MeO means metal oxide) multi-layer film substrate:
Laminated substrate comprising alternating PI and MeO layers would ameliorate the softness and reduce the TCE of PI substrate. However, the lamination process is too complicated. Until now, solar cells with polymer substrate and metal substrate are mostly fabricated by a roll-to-roll process to deposit the various layers on the flexible substrate in vacuum. The equipment, such as the thermal evaporator sputter or the E-Beam gun evaporator, required for depositing the metal or the transparent conductive oxide films on substrate, although having been commercialized, is very expensive. And it is complicated and difficult to adapt to roll-to-roll plasma-enhanced chemical vapor deposition (PECVD), also called glow discharge chemical vapor deposition (GD-CVD) required for depositing amorphous silicon films. Presently, no commercialized PECVD equipment available is used for the roll-to-roll operation. Users must develop the equipment required by themselves. To enhance the efficiency of the solar cells, the N, I, P layers of amorphous silicon would have to be deposited separately in three different reaction chambers (i.e. in a multichamber PECVD equipment) to avoid contaminating each other. It's very difficult to make such solar cells by the roll-to-roll process, which is still under studies now.
Therefore, the following drawbacks exist in depositing the films of polymer solar cells by roll-to-roll process: 1) The available equipment for the glass substrate solar cells cannot be utilized, and the required new equipment is not available; 2) Because the film deposition occurs in a large area, it is difficult to maintain the required flatness of the polymer substrate in this large area film deposition process. It is also difficult to control the deformation of the substrate caused by heating, resulting in expansional deformation of the films deposited and poor quality; 3) It is difficult to perform multichamber with PECVD to increase the efficiency of polymer solar cells; 4) The equipment is expensive, and consequently the production cost is high.
The kapton from Du Pont, as mentioned earlier, has a high thermal expansion coefficient and has strong adhesion between the glass substrate and the PI film deposited thereon, rendering it difficult to have the PI peeled off the substrate.
It is therefore attempted by the Applicant to deal with the above shortcomings encountered by the prior art.